The present invention pertains to a charge pump booster circuit for supplying a voltage different from a source voltage and a voltage supply circuit containing a driver circuit for driving the charge pump booster circuit.
Usually, a DCxe2x80x94DC converter equipped with a switching power source or a charge pump type booster circuit is used to generate a voltage having a level different from that of a source voltage. A voltage higher than the source voltage or a negative voltage can be generated using these circuits.
For example, the frequency range of the TV tuner of a TV receiver can be adjusted (tuning) by changing the tuning voltage applied to the variable capacitor (variable capacity element) of the voltage control oscillator circuit (VCO) according to the channel to be received. A voltage of 30 V or so may be needed for said tuning voltage depending on the frequency band of the signal received.
In recent years, TV tuners have been used widely for automobile TVs and personal computers (personal computer), for example, where compactness, light weight, and portability are often demanded. Thus, a system is needed in which the tuning voltage needed for the TV tuner is generated inside of the device instead of supplied from the outside. A charge pump booster circuit is widely utilized as a voltage generator circuit to this end.
FIG. 4 shows an example configuration of a popular charge pump type booster circuit. As illustrated, said booster circuit is configured with oscillator circuit 10, charge pump driver circuit 20, and charge pump circuit 30. Configurations of the respective partial circuits and their functions will be explained below.
As shown in the figure, oscillator circuit 10 is usually configured with crystal oscillator XTL, capacitor C1, and oscillation amplifier OSC.
Charge pump driver circuit 20 is configured with comparator CMP and buffers BUF1 and BUF2 which supply driving currents IS1 and IS2 to charge pump circuit 30 according to the output signal of comparator CMP.
Charge pump circuit 30 is configured with multi-stage diodes D1, D2, . . . , Dn connected in series between source voltage VCC supplying terminal T1 and output terminal T2, multiple capacitors Cp1, Cp2, . . . provided to serve as a charge pump, output capacitor COUT, and multi-stage Zener diodes ZD1, . . . , ZDm connected in series between output terminal TOUT and ground potential GND.
Capacitors Cp1, Cp2, . . . for the charge pump are connected to output terminals of diodes D1, D2, . . . at one end, and their other ends are connected to output terminals of buffers BUF1 and BUF2 alternately.
FIG. 5 is a circuit diagram showing the internal configurations of oscillator circuit 10 and charge pump driver circuit 20.
As illustrated, oscillator circuit 10 is configured with crystal oscillator XTL, capacitor C1, and oscillation amplifier OSC; and oscillation amplifier OSC is further configured with npn transistors P3 and P4, capacitors C2 and C3, and resistor elements R1 through R6.
Crystal oscillator XTL and capacitor C1 are connected in series between oscillation signal output terminal T3 and ground potential GND.
In addition, npn transistors P1 and P2 diode-connected between the feed line of source voltage VCC and node ND1 are connected in series.
In oscillation amplifier OSC, the base of transistor P3 is connected to terminal T3, its collector is connected to ND1, and the emitter is grounded via resistor element R3. In addition, capacitors C2 and C3 are connected in series between terminal T3 and ground potential GND, and the junction of capacitors C2 and C3 is connected to the emitter of transistor P3.
In addition, resistor elements R4 and R5 are connected in series between node ND1 and ground potential GND, the collector of transistor P4 is connected to node ND1, the base is connected to the junction of resistor elements R4 and R5, and the emitter is grounded via resistor element R6.
Oscillator circuit 10 with the configuration is oscillated at an oscillation frequency unique to crystal oscillator XTL, and oscillation voltage Vosc is output from terminal T3. In addition, the gain of said oscillator circuit is determined based on the capacitances of capacitors C2 and C3.
In charge pump driver circuit 20, the differential circuit comprising npn transistors P5 and P6 and the differential circuit comprising npn transistors P8 and P9 constitute comparator CMP shown in FIG. 4. In addition, pnp transistors Q1 and Q2 and npn transistors P11, P12, P13, and P14 constitute buffers BUF1 and BUF2, respectively.
In the charge pump driver circuit 20, comparator CMP compares oscillation voltage VOSC of terminal T3 of oscillator circuit 10 and the base voltage of transistor P4, generates an oscillation signal according to the result of said comparison, and outputs it to buffers BUF1 and BUF2. As a result, charge pump driving currents IS1 and IS2 having inverted phases with respect to each other are output from buffers BUF1 and BUF2.
Driving currents IS1 and IS2 output from buffers BUF1 and BUF2 are output into capacitors Cp1, Cp2, . . . provided in charge pump circuit 30. Thus, capacitors Cp1, Cp2, . . . are discharged and recharged alternately repeatedly at charge pump circuit 30, so that a voltage higher than source voltage VCC is output to output terminal TOUT. Furthermore, voltage VOUT of output terminal TOUT is smoothened by output capacitor COUT and sustained at a desired voltage by multi-stage Zener diodes ZD1, . . . , ZDm connected in series.
In the charge pump booster circuit, the number of boosting steps is decided according to source voltage VCC and desired output voltage VOUT. Furthermore, in general, the number of boosting steps is set so as to supply a boosted voltage higher than desired output voltage VOUT in order to assure sufficient current driving performance for a load circuit, and the current driving performance of the boosting circuit can be assured by regulating output voltage VOUT at a desired voltage value.
In the conventional charge pump type booster circuit, another feedback loop is formed in charge pump driver circuit 20 via the source impedance in addition to the feedback loop of oscillator circuit 10. In particular, when the impedance of the source line is high, parasitic oscillation is induced by the feedback loop of charge pump driver circuit 20. As a result, oscillator circuit 10 can no longer perform a normal oscillation operation, and the frequencies of the driving currents output from buffers BUF1 and BUF2 are determined by the oscillation frequency of the parasitic oscillation.
Normally, the oscillation frequency of oscillator circuit 10 is controlled by the frequency unique to crystal oscillator XTL. Because the buffers and charge pump circuit 30 are designed in accordance with the oscillation frequency of the oscillator circuit and the oscillation frequency of the parasitic oscillation is determined based on the characteristic of the feedback loop which induces the parasitic oscillation, the parasitic oscillation is considered to oscillate at an oscillation frequency different from the oscillation frequency of the crystal oscillator XTL. Thus, sufficient current can no longer be supplied to charge pump circuit 30, or the frequency of the driving currents becomes either lower or higher than the reference value required by charge pump circuit 30, and voltage VOUT output from charge pump circuit 30 cannot reach the desired voltage, resulting in a disadvantage that the desired current cannot be supplied to the load circuit.
The present invention was created in the light of such situation, and its objective is to present a voltage supply circuit by which the parasitic oscillation of the charge pump driver circuit can be restrained, the charge pump driving currents can be generated at a stable oscillation frequency, and a desired boosting voltage can be supplied to the load circuit.
In accordance with one aspect of the present invention, the voltage supply circuit of the present invention has an oscillator circuit which outputs an oscillation signal having a prescribed frequency, a first comparator circuit which compares the oscillation signal with a prescribed reference signal and outputs a signal in accordance with the result of said comparison, a second comparator circuit which sustains its output at a prescribed level when the amplitude of the output signal of the first comparator circuit is lower than a prescribed value and outputs a signal in accordance with the output signal of the first comparator circuit when the output signal of the first comparator circuit has exceeded the reference value, a buffer circuit which outputs a first driving current and a second driving current having inverted phases with respect to each other according to the output signal of the second comparator circuit, and a charge pump circuit having multiple capacitors to be charged alternately by the first and the second driving currents and which outputs a voltage different from a source voltage.
In addition, in another aspect of the present invention, ideally, the oscillator circuit has a crystal oscillator, generates the oscillation signal with a frequency unique to said crystal oscillator, and outputs it to the first comparator circuit.
In addition, in a further aspect of the present invention, ideally, the first comparator circuit is configured with a differential circuit in which the oscillation signal is input to an input terminal provided on one side, and the reference signal is input to an input terminal provided on the other side.
Furthermore, in yet another aspect of the present invention, ideally, the second comparator circuit is configured with a differential circuit having a hysteresis characteristic.